The spine includes a series of joints routinely called motion segment units, which is the smallest component of the spine that exhibits kinematic behavior characteristic of the entire spine. The motion segment unit is capable of flexion, extension, lateral bending and translation. The components of each motion segment unit include two adjacent vertebrae and their apophyseal joints, the intervertebral disc, and the connecting ligamentous tissue. Each component of the motion segment unit contributes to the mechanical stability of the joint.
Components of a motion segment that move out of position or become damaged can lead to serious pain and may lead to further injury to other components of the spine. Depending upon the severity of the structural changes that occur, treatment may include fusion, discectomy, or laminectomy.
Underlying causes of structural changes in the motion segment unit leading to instability include trauma, degeneration, aging, disease, surgery, and the like. Thus, rigid stabilization of one or more motion segment units may be an important element of a surgical procedure in certain cases (e.g., injuries, deformities, tumors, etc.), whereas it is a complementary element in others (e.g., fusion performed due to degeneration). The purpose of rigid stabilization is the immobilization of a motion segment unit Rigid stabilization typically results in a rigid, internal fixation of all or part of intervertebral joints and usually involves metallic rods, screws, plates, and the like for stabilization. In general, the devices are intended to immobilize the motion segment.
In addition to a loss of mobility, total immobilization of the motion segment also can cause unloading of the disk. This can undesirably impact fusion, for example, slowing or even reducing the growth of bone into our through an implant placed into the disc space. Additionally, unloading of the disc can lead to further degeneration of the disk in the immobilized motion segment. Another drawback is that total immobilization also can cause the mobility of the motion segment to be transferred to other motion segments of the spine. The added stresses transferred to motion segments neighboring or nearby the immobilized segment can cause or accelerate the degeneration of those segments.
Thus, there is a need for improved systems that can stabilize motion segments with reduced degeneration of neighboring joints with faster and more substantial fusion.